Brake proportioning valve with blend back

ABSTRACT

A proportioning valve located in the hydraulic system of a vehicle to control the braking force on the rear wheels with respect to that on the front wheels. The valve includes a housing which is provided with an inlet chamber connected to one line of the vehicle&#39;&#39;s dual master cylinder, and an outlet chamber in communication with the rear brakes of the vehicle. The valve is normally open, permitting unimpeded fluid flow between the inlet and outlet chambers, during an initial brake application. However, when the pressure level in the outlet chamber reaches a first value, the valve reacts to restrict the flow between the two chambers so that the pressure level in the inlet chamber rises more rapidly than the pressure level in the outlet chamber up to a second value of pressure level within the outlet chamber. Upon attaining the second value of pressure level, the valve is acted upon by a pressure responsive member which causes the pressure level in the outlet chamber to increase at a greater rate than the pressure level in the inlet chamber. This pressure level in the outlet chamber continues to build up at this new accelerated rate until the chamber pressures are substantially the same, at which time the valve opens to establish substantially uninterrupted flow between the chambers.

United States Patent [1 1 Fulmer [.1] 3,790,221 Feb.5,1974

[ BRAKE PROPORTIONING VALVE WITH BLEND BACK [75] Inventor: Keith H.Fulmer, South Bend, Ind.

[73] Assignee: The Bendix Corporation, South Bend, Ind.

[22] Filed: Aug. 24, 1972 [21] Appl. No.: 272,265

[52] US. Cl. 303/6 C, 188/349 [51] Int. Cl B60t 8/26 [58] Field ofSearch 303/6 C, 84 A, 84 R;

188/151 A, 152, 349; 60/545 E; 340/52 C; 200/82 D; 137/508, 493.9

[56] References Cited UNITED STATES PATENTS 3,492,052 l/l970 Klimek303/6 C 3,612,618 10/1971 Swanson 303/6 C Primary Examiner-Duane A.Reger Assistant Examiner-D. C. Butler Attorney, Agent, or Firm-Leo H.McCormick, Jr.; William N. Antonis [57] ABSTRACT A proportioning valvelocated in the hydraulic system of a vehicle to control the brakingforce on the rear wheels with respect to that on the front wheels. Thevalve includes a housing which :is provided with an inlet chamberconnected to one line of the vehicles dual master cylinder, and anoutlet chamber in communication with the rear brakes of the vehicle. Thevalve is normally open, permitting unimpeded fluid flow between theinlet and outlet chambers, during an initial brake application. However,when the pressure level in the outlet chamber reaches a first value, thevalve reacts to restrict the flow between the two chambers so that thepressure level in the inlet chamber rises more rapidly than the pressurelevel in the outlet chamber up to a second value of pressure levelwithin the outlet chamber. Upon attaining the second value of pressurelevel, the valve is acted upon by a pressure responsive member whichcauses the pressure level in the outlet chamber to increase at a greaterrate than the pressure level in the inlet chamber. This pressure levelin the outlet chamber continues to build up at this new accelerated rateuntil the .chamber pressures are substantially the same, at which 6Claims, 2 Drawing lFigures BACKGROUND OF THE INVENTION This inventionrelates to a proportioning valve which is used in a vehicle hydraulicbraking system for maintaining a braking force relationship between thefront and rear brakes.

Numerous innovations have been employed in the automotive vehicle brakeart to cope with the weight shift from the rear wheels to the frontwheels when the vehicle is decelerated. The weight shift to the frontwheels being caused by the vehicle attempting to rotate about its centerof mass when braked. One approach to meeting this problem involved theuse of larger brakes on the front wheels. However, this only improvedfront end braking but did not take care of the wheel lock up phenomenon,occurring at the rear wheels during a normal brake application, causedby weight shifting of the vehicle. With the present day trend to frontwheel disc brakes, the lock up problem was intensified and, in fact,motivated the use of proportioning valves in the hydraulic lines to therear wheel brakes to reduce the brake applying pressure. The use ofproportioning valves to reduce the brake applying pressure on the rearwheel brakes does prevent lock up of the brakes. Unfortunately, however,many of these valves will continue to reduce the braking pressure to therear wheel brakes even though brake fade has set in and higher pressurelevels in the vehicles master cylinder are available for braking, thusdepriving the rear wheel brakes of their full braking capacity in theevent of brake fade." Brake fade is characteristic of drum brakes whenoperated under high temperatures. Under faded" conditions considerablyhigher braking pressures are required to move the shoes into the drumsto effectively perform a braking function. This invention overcomes thedeficiencies of prior art proportioning valves by utilizing the maximummaster cylinder pressure upon occurence of brake fade to achieve themost effective braking.

SUMMARY OF THE INVENTION A proportioning valve for the rear wheel drumbrakes of a vehicle responsive to an increase in the valve outletpressure above a predetermined value to admit maximum master cylinderpressure to the brakes to insure maximum effective braking at all times.

A proportioning valve located in the hydraulic line to the rear wheeldrum brakes of a vehicle responsive to master cylinder pressure sensedat the valve outlet above a predetermined value for overruling theproportioning effect of the valve to permit the application of higherpressures at the rear brakes under adverse braking conditions.

A control mechanism for the rear wheel drum brakes of a vehicleincluding a proportioning valve responsive to a predetermined pressurelevel at its outlet for maintaining a lower pressure level at the rearbrakes than that pressure developed at the master cylinder until thepressure level at the outlet attains a' given value, at which time theproportioning effect of the valve ceases and the valve is connected forsubstantially uninterrupted fluid flow from inlet to outlet.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of avehicle braking system incorporating a proportioning valve constructedin accordance with the invention shown in longitudinal section; and

FIG. 2 is a graphical representation of the pressure levels appearing atthe inlet and outlet of the proportioning valve of FIG. 1 under varyingmaster cylinder pressures.

DETAILED DESCRIPTION Referring now to FIG. 1 of the drawing, thereference numeral 10 designates a dual master cylinder, havingindependent conduits l2 and 14, communicating respectively, with discfront brakes l6 and drum rear brakes 18'through a brake controlmechanism 20. The control mechanism 20 includes proportioning valvemeans 22 and a brake failure warning device 24 located in a stepped bore26 of housing 28. A switch actuating piston 30 of the brake failurewarning device 24 is reciprocally positioned in the smaller diameterportion of the stepped bore 26 so as to sense: the dual master cylinderpressures in communication with the brake control mechanism via lines 12and 14. Brake failure warning devices are well known and since it formsno part of the present invention no further explanation or descriptionis believed necessary. For a complete dissertation on warning devices ofthe type shown herein, reference is made to US. Pat. No. 2,556,607,owned by the assignee of the present invention.

The proportioning valve 22, which is located in the larger diametersection of the bore 26, divides that section of the bore into chambers32 and 34, communicating respectively, with inlet port 36 and outletport 38, the latter port being connected to the drum brakes 18. Thevalve 22 includes a plunger or piston 40 slidably mounted in an endclosure member 42 of the bore for axial movement therein. The end member42 is secured in the bore 26 between a cup-shaped member 44 and a plug46 threadedly attached to the open end of the bore 26. Channels 48 inthe cup-shaped member 44 establish communication between the inlet 36and the inlet chamber 32. The piston 40 is provided with a smallerdiameter end 50 exposed to the inlet chamber 32 and a larger diameterend 52 exposed to the outlet chamber 34. Accordingly, the piston 40 hasa smaller effective area exposed to the inlet chamber and a largereffective area exposed to the outlet chamber. An abutment 54, formedbetween the ends 50 and 52 of the piston 40 engages an annular portion56 of the member 42 to thereby limit piston movement to the left intothe chamber 32. Displacement of the piston 40 to the right, as viewed inFIG. 1, is determined by engagement of the larger end 52 of the pistonwith the plug 46. The end plug 46 and the larger end 52 of the piston 40form the outlet chamber 34. The piston 40 is urged to the right end ofthe bore against the plug 46 by a spring 58 which is interposed betweena flange 60 of the cupshaped member 44 and a flat annular member 62.,having an opening 64 into which the smaller end 50 of the piston 40fits. The piston 40 is formed with a central passage 66, communicatingthe inlet chamber 32 with the outlet chamber 34. The valve 22 furtherincludes a poppet or valve member 68 reciprocally mounted to a stem 70which is carried on one end of the piston 30 of the warning device 24. Aspring 72 urges the valve member 68 against a radially inward extendingflange 74 of a perforated retainer element 76. A radially outwardextending flange 78 of the retainer element 76 is disposed between thespring 58 and the flange 60. The valve member 68 is located on the stem70 in axial alignment with the piston 40 and the central passage 66 tocontrol fluid flow through the passage and to regulate the pressurelevels in the chambers 32 and 34. A valve seat 71 is formed at that endof the passage 66 which terminates adjacent the inlet chamber 32 forengagement by the valve member 68.

' Should the braking condition of the vehicle be such that it becomesexpedient to render brake proportioning to the rear brakes ineffective,for example, during brake fade, additional means 80 is associated withthe proportioning valve to perform this function. That is, means 80 isprovided for overruling the action of the porportioning valve andsubjecting the rear brakes to higher pressure levels by blending thethen existing pressure level of the rear brakes into the higher levelsof the master cylinder. The means 80 includes an internal piston orplunger 82 which is formed with a longitudinal opening 83 in registrywith the passage 66. The piston 82 is concentrically located within thepassage 66 of the valve piston 40 for sliding movement between a steppedshoulder 84 of the piston 40 and a retainer ring 86, the latter beingpositioned at the larger diameter end 52 of the piston 40. A spring 88is preloaded between the stepped shoulder 84 and the internal piston 82,urging the piston 82 against the retainer ring 86. The internal piston82 is stepped and is further provided with an annular end 90 of lessereffective area than annular end 92. Accordingly, fluid pressure withinthe central passage 66 and the outlet chamber 34 acts on these annularends 90 and 92 respectively so that the net force developed by the fluidpressure tends to move the internal piston 82 to the left, as viewed inFIG. 1, against the preload of the spring 88. A pin or finger 94 ismounted to the central portion of the internal piston 82 and projectsalong the central passage 66, terminating short of engagement with, butadjacent to the valve member 68. The relationship of the pin 94 withrespectto engagement of thevalve member 68 is such that when theinternal piston 82 is urged against the retainer ring 86 the seat 71 ofthe piston 40 seats and unseats freely, without interference from thepin element 94, on the valve member 68 in proportioning the brakingforce of the rear brakes 18.

MODE OF OPERATION With reference to FIG. 1, the components of the brakecontrol mechanism are illustrated in the position they occupy duringrelease of the vehicle brakes. To brake the vehicle, the foot pedal ofthe master cylinder 10 is depressed, developing hydraulic fluid pressurewhich is communicated to the inlet port 36 and the chamber 32 of thevalve housing 28. Since the valve member 68 is now unseated from theseat 71 located at the smaller end 50 of the piston 40, the pressurizedfluid in the chamber 32 causes fluid to flow through the central passage66 and the outlet chamber 34 to the rear brakes 18. At this time, duringthe initial brake applying period with the valve member 68 out ofengagement with the seat 71 on the piston 40, the fluid pressure in theinlet chamber 32 is the same as the pressure in the outlet chamber 34 upto a given pressure value, such as point A, FIG. 2. This may be seen byreference to the line OA on the inlet-outlet pressure graph of FIG. 2.The same fluid pressure which appears in both chambers 34 and 32respectively, acts on the entire area of the larger diameter end 52 ofthe piston 40, shown as X and on the entire area of the smaller diameterend 50, shown as X This, of course, creates a net force unbalancetending to displace the valve piston 40 to the left, as viewed inFIG. 1. As the master cylinder pressure continues to rise in thechambers 32 and 34, the fluid pressure in the outlet chamber 34 reachesa predetermined value A, as shown on the graph of FIG. 2, at which thenet force acting on the piston 40 is sufficient to overcome the spring58, displacing the piston to the left in FIG. 1, closing offcommunication through passage 66. Further increasing the fluid pressurelevel in the inlet chamber 32 subsequently unseats the piston 40 fromthe valve element 68 to thereby establish communication with the centralpassage 66, outlet chamber 34, and the rear brakes 24. This produces arise in pressure at the outlet chamber. The rise in magnitude in thefluid pressure level in the outlet chamber 34, however, is only afraction of the magnitude of the fluid pressure rise in the inletchamber 32 before the valve closes again, since the area embraced by thediameter X of the piston 40 is greater than the area embraced by thediameter X located at the smaller end 50 of the piston which is exposedto inlet chamber pressure. This seating and unseating of the valvepiston 40 and the valve member 68 continues as the pressure level risesin the inlet chamber 32, maintaining the pressure levels in the twochambers 32 and 34 in the ratio of the areas X, and X with the pressurelevel in the outlet chamber 34 always less, as represented by line AB,up to point B on the curve of FIG. 2.

When the pressure in outlet chamber 34 attains a predetermined pressurelevel, such as point B on the graph in FIG. 2, the fluid pressure actingon the annular ends of the internal piston 82 is now of such magnitudethat the net force on the piston 82 moves the piston to the left, inFIG. 1, against the spring 88. This movement of the piston 82 positionsthe pin 94 so that its end engages the valve member 68, preventing thevalve from seating on seat 71 and completely closing off communicationbetween the chambers. As the fluid pressure continues to rise in theoutlet chamber 34, the internal piston 82 and pin 94 move farther to theleft, thus creating a greater gap between the valve member 68 and theseat 71 located in the end of the valve piston 40. Since the outletchamber 34 is now continuously open in varying amounts, via the gapbetween the member 68 and the end of valve piston 40, to the inletchamber 32, the fluid pressure rise in the outlet chamber 34 is at agreater rate than the rise in the inlet chamber 32, until the point C onthe curve in FIG. 2 is reached. Observe that from point B to point C onthe curve of FIG. 2, the proportioning action of the valve means, i.e.maintaining the braking force on the rear brakes within a given range,has been overruled. At point C on the curve FIG. 2, the internal piston82 has moved to the left so that the annular end abuts the steppedshoulder 84 of the piston 40, forcing the pin 94 out of the open end ofthe central passage 66 so as to further dis place the valve member 68,permitting uninterrupted fluid communication, between the chambers 32and 34 and the rear brakes 18. Any further brake pressure increasebeyond point C will cause the pressure levels in the chambers 32 and 34to rise at about the same rate, since the valve means is open.

, Upon release of the vehicles brakes, the pressure levels within thechambers 32 and 34 will theoretically follow the curve as shown in FIG.2, allowing for hysteresis losses, etc. of the component parts of thevalve means. Initial brake release reduces the pressure level in theinlet chamber 32 until the pressure difference in the chambers is suchas to cause the valve member 68 to readjust its relationship to the pin94 and consequently its position with respect to the seat 71 in the end50 of the piston 40, indicated by point D in FIG. 2. Then the valvemeans 22 in conjunction with its means 80 meters fluid under pressurebetween the chambers 32 and 34 substantially along the curve DE of FIG.2. At point E the spring 88 has moved the internal piston 82 and the pin94 to the right so that the valve member 68 can seat on the seat 71 inthe end of the passage 66 of the piston 40. A further reduction in inletchamber pressure at this point meters fluid along the curve EF as thevalve member 68 seats and unseats on the end 50 of the piston under theinfluence of the net force acting on the piston 40 which net forceincludes the pressure differential acting across the valve piston 40 andthe force of springs 58 and 72, applied to the valve piston. As theinlet chamber pressure further decays, the spring 58 positions the valvepiston 40 against the housing plug 46, as shown in FIG. 1, causing thepressure level in the chambers 32 and 34 to drop along the curve F to Gto O. I

I claim:

1. In a brake control mechanism for use in a vehicle hydraulic system:

a housing having an inlet port, an outlet port and a bore communicatingthe inlet port with the outlet port;

valve means disposed within the bore and dividing the bore into an inletchamber and an outlet chamber, said inlet and outlet chamberscommunicating respectively, with the inlet and outlet ports, said valvemeans including a valve member in the inlet chamber, a valve pistonreciprocably positioned in the bore and having a passage thereinconnecting the inlet and outlet chambers for fluid communication, avalve seat located at that end of the passage which terminates in theinlet chamber for engagement with the valve member, said valve pistonhaving a smaller end of lesser effective area exposed to inlet chamberpressure and a larger end of greater effective area exposed to outletchamber pressure, said valve piston having an open position whereinfluid communication is established between the chambers until a firstfluid pressure level in the outlet chamber is reached whereupon thevalve piston responds to the first fluid pressure level toproportionally restrict fluid flow between the chambers to permit thefluid pressure level in the inlet chamber to rise more rapidly than thefluid pressure level in the outlet chamber rises until a second fluidpressure level in the outlet chamber is reached;

first resilient means biasing said valve piston in a direction away fromsaid valve member to establish the aforesaid open position; and

additional means in the passage of said valve piston 6 responsive to thesecond pressure level in the outlet chamber for lifting the valve memberfrom the valve seat and maintaining a varying gap between the seat andvalvemember so that the fluid pressure level in the outlet chamber nowrises more rapidly than the fluid pressure level. in the inlet chamberupto a pressure level in the outlet chamber substantially the same as thatin the inlet chamber whereupon said valve means opens to permituninterrupted flow of fluid between the chambers.

2. The invention of claim 1:

said valve piston having a fluid passage therein communicating the inletchamber with the outlet cham ber;

said valve member being positioned adjacent the smaller end of the valvepiston to open and close the passage to effectively regulate fluid flowthrough the passage.

3. The invention of claim 1:

said additional means including a stepped piston having opposed annularends and an annular shoulder between the ends, one of the ends having aneffective area greater than the effective area of the other end, saidopposed ends being acted upon by the pressure level appearing in theoutlet chamber, a coil spring in the passage having one end abutting thevalve piston and the other end abutting the shoulder on the steppedpiston to urge the latter toward a first position so that the pressurein the outlet chamber which acts on the opposed ends of the steppedpiston establishes a net force on the piston opposing the force of thecoil spring and urges the piston to a second position, and a fingercarried by the stepped piston for engagement with the valve member tounseat the same upon movement of the stepped piston to the secondposition.

4. The invention of claim 1:

said additional means including an internal piston coaxially located inthe passage of the valve piston and slidable between a first and asecond position;

second resilient means in the passage urging the internal piston towardthe first position;

said internal piston having an axial opening therethrough and opposedannular ends, one of the ends having an effective area greater than theeffective area of the other annular end, said opposed ends being actedupon by the pressure level existing in the outlet chamber, the pressurein the outlet chamber acts on the ends of the internal piston toestablish a net force on the piston tending to slide the piston againstthe second resilient means and toward the second position; and

an element carried by the internal piston and extending along thepassage for engagement with the valve member upon movement of theinternal piston to its second position.

5. The invention of claim 4:

said second resilient means being a coil spring having opposite endsengaging respectively, the valve piston and the internal piston;

said element being in the form of a pin extending along the passagetoward the smaller diameter end of the valve piston and through thecoils of the spring. a

6. In a brake control mechanism for a vehicle hydraulic system: I

a housing having an inlet port, an outlet port and a bore communicatingthe inlet port with the outlet port;

valve means in the bore dividing the bore into an inlet said valve meansincluding a valve piston slidably arranged in the bore and provided witha passage therethrough communicating the inlet and outlet chambers, aseat in the passage, a valve member slidably located in the bore forseating on the seat, an internal piston slidably positioned in thepassage of the valve piston and having a longitudinal opening inregistry with the passage, and a pin element mounted in the longitudinalopening of the internal piston and extending through the passage towardthe seat for engagement with the valve member to prevent seating;

said valve piston having a smaller diameter end of lesser effective areaexposed to fluid pressure in the inlet chamber and a larger diameter endof greater effective area exposed to fluid pressure in the outletchamber, said valve piston being spring biased to a first positionunseating the valve piston from the valve member;

said internal piston having opposed annular ends, one

annular end having an effective area greater than the effective area ofthe other annular end, said intemal piston being spring biased in adirection causing withdrawal of the pin element from engagement with thevalve member, said annular ends being exposed to outlet chamber pressureso that the net force developed on the opposed annular ends of theinternal piston acts so as to move the internal piston against thespring bias;

said valve piston being responsive to a predetermined pressure level inthe outlet chamber to seat said piston against the valve member andsubsequently regulate the fluid flow between the chambers so that thefluid pressure level in the inlet chamber rises more rapidly than thefluid pressure level in the outlet chamber rises until a secondpredetermined pressure level in the outlet chamber is reached whereuponthe spring biased internal piston is moved in a direction which drivesthe pin element against the valve member to unseat the latter so thatthe fluid pressure level in the outlet chamber rises more rapidly thanthe fluid pressure level in the inlet chamber until the fluid pressurelevel in the outlet chamber is almost the same as the fluid pressurelevel in the inlet chamber, at which time the valve piston and internalpiston have moved the valve member to its maximum open position.

1. In a brake control mechanism for use in a vehicle hydraulic system: ahousing having an inlet port, an outlet port and a bore communicatingthe inlet port with the outlet port; valve means disposed within thebore and dividing the bore into an inlet chamber and an outlet chamber,said inlet and outlet chambers communicating respectively, with theinlet and outlet ports, said valve means including a valve member in theinlet chamber, a valve piston reciprocably positioned in the bore andhaving a passage therein connecting the inlet and outlet chambers forfluid communication, a valve seat located at that end of the passagewhich terminates in the inlet chamber for engagement with the valvemember, said valve piston having a smaller end of lesser effective areaexposed to inlet chamber pressure and a larger end of greater effectivearea exposed to outlet chamber pressure, said valve piston having anopen position wherein fluid communication is established between thechambers until a first fluid pressure level in the outlet chamber isreached whereupon the valve piston responds to the first fluid pressurelevel to proportionally restrict fluid flow between the chambers topermit the fluid pressure level in the inlet chamber to rise morerapidly than the fluid pressure level in the outlet chamber rises untila second fluid pressure level in the outlet chamber is reached; firstresilient means biasing said valve piston in a direction away from saidvalve member to establish the aforesaid open position; and additionalmeans in the passage of said valve piston responsive to the secondpressure level in the outlet chamber for lifting the valve member fromthe valve seat and maintaining a varying gap between the seat and valvemember so that the fluid pressure level in the outlet chamber now risesmore rapidly than the fluid pressure level in the inlet chamber up to apressure level in the outlet chamber substantially the same as that inthe inlet chamber whereupon said valve means opens to permituninterrupted flow of fluid between the chambers.
 2. The invention ofclaim 1: said valve piston having a fluid passage therein communicatingthe inlet chamber with the outlet chamber; said valve member beingpositioned adjacent the smaller end of the valve piston to open andclose the passage to effectively regulate fluid flow through thepassage.
 3. The invention of claim 1: said additional means including astepped piston having opposed annular ends and an annular shoulderbetween the ends, one of the ends having an effective area greater thanthe effective area of the other end, said opposed ends being acted uponby the pressure level appearing in the outlet chamber, a coil spring inthe passage having one end abutting the valve piston and the other endabutting the shoulder on the stepped piston to urge the latter toward afirst position so that The pressure in the outlet chamber which acts onthe opposed ends of the stepped piston establishes a net force on thepiston opposing the force of the coil spring and urges the piston to asecond position, and a finger carried by the stepped piston forengagement with the valve member to unseat the same upon movement of thestepped piston to the second position.
 4. The invention of claim 1: saidadditional means including an internal piston coaxially located in thepassage of the valve piston and slidable between a first and a secondposition; second resilient means in the passage urging the internalpiston toward the first position; said internal piston having an axialopening therethrough and opposed annular ends, one of the ends having aneffective area greater than the effective area of the other annular end,said opposed ends being acted upon by the pressure level existing in theoutlet chamber, the pressure in the outlet chamber acts on the ends ofthe internal piston to establish a net force on the piston tending toslide the piston against the second resilient means and toward thesecond position; and an element carried by the internal piston andextending along the passage for engagement with the valve member uponmovement of the internal piston to its second position.
 5. The inventionof claim 4: said second resilient means being a coil spring havingopposite ends engaging respectively, the valve piston and the internalpiston; said element being in the form of a pin extending along thepassage toward the smaller diameter end of the valve piston and throughthe coils of the spring.
 6. In a brake control mechanism for a vehiclehydraulic system: a housing having an inlet port, an outlet port and abore communicating the inlet port with the outlet port; valve means inthe bore dividing the bore into an inlet chamber and an outlet chamber,said inlet and outlet chambers communicating respectively, with theinlet port and the outlet port; said valve means including a valvepiston slidably arranged in the bore and provided with a passagetherethrough communicating the inlet and outlet chambers, a seat in thepassage, a valve member slidably located in the bore for seating on theseat, an internal piston slidably positioned in the passage of the valvepiston and having a longitudinal opening in registry with the passage,and a pin element mounted in the longitudinal opening of the internalpiston and extending through the passage toward the seat for engagementwith the valve member to prevent seating; said valve piston having asmaller diameter end of lesser effective area exposed to fluid pressurein the inlet chamber and a larger diameter end of greater effective areaexposed to fluid pressure in the outlet chamber, said valve piston beingspring biased to a first position unseating the valve piston from thevalve member; said internal piston having opposed annular ends, oneannular end having an effective area greater than the effective area ofthe other annular end, said internal piston being spring biased in adirection causing withdrawal of the pin element from engagement with thevalve member, said annular ends being exposed to outlet chamber pressureso that the net force developed on the opposed annular ends of theinternal piston acts so as to move the internal piston against thespring bias; said valve piston being responsive to a predeterminedpressure level in the outlet chamber to seat said piston against thevalve member and subsequently regulate the fluid flow between thechambers so that the fluid pressure level in the inlet chamber risesmore rapidly than the fluid pressure level in the outlet chamber risesuntil a second predetermined pressure level in the outlet chamber isreached whereupon the spring biased internal piston is moved in adirection which drives the pin element against the valve member tounseat the latter so that the fluid pressure level in the outlet chamberrises more rapIdly than the fluid pressure level in the inlet chamberuntil the fluid pressure level in the outlet chamber is almost the sameas the fluid pressure level in the inlet chamber, at which time thevalve piston and internal piston have moved the valve member to itsmaximum open position.